Nondestructive evaluation (NDE) technologies have recently been challenged to inspect thick, layered, conducting materials for fatigue and corrosion damage. Structures that fall into this class, such as airframe wings, pose significant difficulties for conventional inspection techniques, and especially challenging is the detection of deeply buried flaws at airframe fasteners. Reflections of ultrasound at layer boundaries cause serious problems for the application of ultrasonic inspection methods. Conventional eddy current inspection techniques are also compromised due to the exponential decay of electromagnetic energy with depth into a conductor.
Eddy current techniques, using detection that is sensitive to the time rate of change of the magnetic field level, are currently the most widely used method for the detection of hidden damage in thin conductors. The sensitivity of the method, however, is severely limited as material thickness increases. Eddy currents decay exponentially with both depth into the material and the square root of the applied frequency. Sufficient field penetration requires the reduction of the excitation frequency. This reduction in frequency, however, limits the sensitivity of the inductive pickup sensor whose output is proportional to the frequency. U.S. Pat. No. 5,648,721, which is hereby incorporated by reference, teaches a flux focusing eddy current probe having separate excitation and pick-up coils, magnetically isolated from one another by a highly permeable flux focusing lens.
The use of Giant Magnetoresistive (GMR) sensors for electromagnetic nondestructive evaluation has grown considerably in the last few years. Technological advances in the research and development of giant magnetoresistive materials has led to commercially available GMR sensors with many qualities well suited for electromagnetic NDE. Low cost GMR magnetometers are now available which are highly sensitive to the magnitude of the external magnetic field, have a small package size, consume little power, and operate at room temperature. Incorporation of these sensors into electromagnetic NDE probes has widened the application range of the field. In particular, the low frequency sensitivity of the devices provides a practical means to perform electromagnetic inspections on thick-layered conducting structures.
There are several other eddy current type methods which use detection sensitive to the magnetic field level as opposed to the time rate of change of the field. Magneto-Optic and Hall sensors are two common techniques. These devices, however, typically require substantial instrumentation and have sensitivities much lower than that of the GMR sensor. Super Conducting Quantum Interface Devices (SQUIDS) have sensitivities better than the GMR devices but the requirement of cryogenic temperatures imposes difficulties in application and typically greatly increases the sensor size. Flux Gate Magnetometers have sensitivities similar to GMR sensors and although they do not require a cryogenic temperature, they do require substantial instrumentation and typically relatively large probe size.
Ultrasonic techniques have been shown to be useful for detecting flaws in thick materials, but have limited success in multilayered structures due to reflection of the wave at the interfaces of the layers. Thermographic techniques have difficulties with metallic and thick materials, and typically require greater amounts of instrumentation for application and data analysis. X-ray methods pose environmental concerns and are not typically portable due to the large instrumentation requirements.
A variety of non-destructive evaluation techniques are currently used to inspect rivet joints, with eddy current testing being the most widely used technique. Several different types of eddy current probes have been developed for the specific purpose of rivet inspection. U.S. Pat. No. 5,648,721, herein incorporated by reference, teaches a probe having a flux focusing lens positioned between an excitation coil and a pick-up coil which are isolated from one another by a flux focusing lens, and a means for rotating the probe about the circular inhomogeneity. The device is very accurate for the detection of near surface fatigue cracks but the performance for deeper inspections is limited due to several features of the design, including flux leakage around the flux focusing lens and decreased detection sensitivity with lowered frequency. Other techniques include a Sliding Probe and a method using pencil probes and templates to trace around the fastener head. The sensititivy of the Sliding Probe is often low and a preferred orientation of the probe may lead to false calls or undetected flaws for rivets which are not aligned in a row. Template methods using pencil probes are very time consuming, and lift off or probe wobble can produce false signals.